The continuing accumulation of scrap tires is a major global environmental hazard. The industrialized world continues to amass used tires at the alarming yearly rate of one for every man, woman, and child.
According to the Rubber Association of Canada, there are 29.8 million scrap tires generated annually in Canada (equating to 37.1 million passenger tire equivalents). This generation comes from both the replacement tire market and vehicles that have been scrapped.
In the United States, the Rubber Manufacturers Association estimates that 299 million scrap tires were generated in 2005. Of this, an estimated 42 million tires were stockpiled in landfills, contributing to a total 188 million tires in total stockpiled across the US (the US EPA estimates the stockpiled amount to be 265 million).
Generally, landfill use is declining while the recycling of tires is growing. Currently, approximately 70% of scrap tires are processed in Canada with the balance being stockpiled or exported. However, these proportions can vary considerably by province. For instance, it is estimated that roughly half of all scrap tires generated in Ontario each year are sent over the US/Canada border to be burned as fuel in the US. In Quebec, somewhere between 30% and 40% of scrap tires each year are sent to privately-owned stockpiles located throughout the province.
Moreover, the demanding product specifications for safe, durable tires make scrap tires difficult and expensive to break down.
Tires, which are generally composed of approximately 65% rubber, 10% fibre and 12.5% steel by weight, can be recycled in two forms: processed and whole. Whole tire recycling involves using the old tire, as is, for other purposes (e.g., landscape borders, playground structures, dock bumpers and highway crash barriers). The recycling of processed tires, on the other hand, requires first reducing the tire to smaller pieces. This can be accomplished by chopping, shredding, or grinding at ambient or cryogenic temperature.
Punching or die cutting small sections of rubber from tire treads or sidewalls can be used to create items such as water troughs. This technique is typically done with non-road tires, such as those used on earth moving or mining equipment, or farm tractors.
The process of shredding and grinding scrap tire rubber, and the shred size, depends upon its intended end use. Possible applications include using shred as a lightweight fill for highway embankments, retaining walls and bridge abutments, and as an insulation to limit the depth of frost penetration beneath roads.
Crumb rubber is produced by either an ambient or cryogenic grinding process. Ambient processing is conducted at room temperature. Cryogenic processing uses liquid nitrogen, or other materials or methods, to freeze the rubber chips or particles prior to further size reduction. Particle sizes range from one-quarter inch to fine powder generally used for producing molded products. Uses for larger sized crumb rubber include safety and cushioning surfaces for playgrounds, horse arenas and walking and jogging paths.
Through the use of heat and pressure and a binder, crumb rubber may be molded into various products. Examples include rubber mats used in skating rinks, roof shakes, and rubber mattresses used in livestock stalls.
The production of energy from tires, although technically not a form of recycling, accounts for a significant proportion of used tire disposal. In this application, scrap tires are used as an alternative to coal for fuel in cement kilns, pulp and paper mills, and industrial and utility boilers. This is especially the case in the United States, where tire-derived fuel (TDF) accounted for approximately 155 million scrap tires in 2005, or about 52% of all scrap tires generated.
The tire recycling market faces challenges in that recycled rubber products often cannot meet the quality of products made from virgin rubber, yet they often are more expensive to make. For example, rubberized asphalt is more expensive than normal asphalt, but has not proved to be superior to it; in fact, many transportation engineers are skeptical of its merits. When it is time to repave a rubberized-asphalt road, the top layer cannot be stripped off, heated and reused, because the heat burns the rubber and releases toxic emissions. In addition, rubberized asphalt consumes 25% more petroleum.
As well, considerable research has gone into rubber devulcanization, whereby recycled tires are used in the production of molded or die cut rubber materials such as mats, tubs, and pails such as mats, tubs, and pails. However, the final renewed material has slightly different chemical properties from virgin rubber, and is more rigid and less flexible. As a result, the recycled material does not meet the stringent requirements of modern tire manufactures, nor can it be used in the manufacture of flexible products such as hoses. As these applications account for 85% of Canada's rubber market, the potential supply of devulcanized rubber tends to exceed demand. In addition, the cost of processing old tires, particularly modern radial tires with steel belts, into devulcanized rubber exceeds the cost of virgin rubber production. As a result of this quality/cost challenge, many rubber recycling enterprises either cannot sustain themselves on a commercially attractive basis, or, worse, cannot prosper without government assistance.
Meanwhile, TDF activity has increased, but this is facing more opposition each year. Firstly due to air quality concerns from the general public and civil society organizations. Burning in cement kilns or incinerators results in high NOx, dioxins, PAH, furans, PCB and heavy metals in particulates (flue dusts). Moreover, the high-tech incinerators needed for such operations are very expensive. To ensure their long-term economic stability, heavily-urbanized regions generating a huge and constant supply of scrap tires are required. A current example of public aversion to TDF is the recent ruling by Ontario Divisional Court to uphold a citizen-led appeal of Lafarge Canada's plan to burn tires and other materials in a cement kiln in Bath, Ontario. The appeal cited concerns about potential air pollution, water contamination, and human health impacts.
Pyrolysis systems refer to the thermal processing of waste in the absence (or near absence) of oxygen. Major component fractions resulting from the pyrolysis of vehicle tires are:                a) a gas stream containing primarily hydrogen, methane, carbon monoxide, carbon dioxide and various other gases. The gas after cleaning is very similar to natural gas with about the same energy content, but with a higher heat content;        b) a liquid fraction of an oil stream containing simple and complex hydrocarbons similar to No. 6 fuel oil; and,        c) a char consisting of almost pure carbon, plus some inert materials (e.g. steel, zinc oxide) originally present in the scrap tire.        
A traditional pyrolysis process involves heating tires under substantially anaerobic conditions so that the tire material is not completely converted to gases and ash. The typical automobile tire contains approximately 4 litres of oil, about 230 g of fibre, a kilogram or more of carbon black and about a kilogram each of steel and methane.
However, despite prior art efforts to commercialize pyrolysis technology, it has not yet been achieved in an economically viable way. Although many pyrolysis projects have been proposed, patented, or built over the past decade, none have been commercially successful. Many of these processes are not truly continuous, but are, in at least some aspects or steps, limited to batch processing techniques. As such, they suffer from not being sufficiently scalable so as to be commercially viable. Others require excessive energy inputs to produce end products of sufficiently high quality to permit recycling, with the result that they are not economical. In particular, the products of batch-type tire pyrolysis have limited marketability due to the low quality of their end products as compared to virgin materials. For instance, prior art pyrolytic carbon black (CBp) typically contains too many contaminants for use in new tires. Moreover, with batch pyrolysis techniques, the consistency of the end products may vary with each run. As such, the resulting CBp cannot compete in the auto, rubber, and other industry sectors, which require consistent a carbon black product. As a result, much of the CBp arising from existing pyrolysis processes are used as high grade coal for the fuel industry, as well as for industrial hoses, mats, roofing materials and moldings.
Accordingly, none of these prior art recycling processes have received the widespread acceptance level necessary to effectively tackle the environmental problem posed by ever-increasing levels of scrap tires.